Field of the Invention
The invention relates to a process for treating linear polymer resins to reduce the molecular weight thereof and is particularly directed to a process for treating linear polymer resins which are composed of repeating hydrocarbon units linked together by ester or amide linkages.
Poly(ethylene terephthalate) resins are commonly used for making clear plastic beverage containers. These containers necessarily are of the throw-away type, as they are not suitable for cleaning and refilling due to sanitary considerations and/or the possibility of polymer degradation during the cleaning process. In principle, the containers could be ground and the granulate cleaned by some means which would not degrade the polymer, and the granulate then used for fabrication of some other article in which sanitary considerations were not pertinent. However, each thermoplastic processing application has its own requirements for polymer processing characteristics, of which the polymer average molecular weight is often the single most important determinant. The average molecular weight of poly(ethylene terephthalate) (PET) used for making beverage containers is relatively high, and is too high for many other applications in which the recovered container polymer might otherwise be used.
Other types of linear polymer resins such as polycarbonate and polyamide resins sometimes present like problems. In the case of polyamides prepared by polymerization of cyclic monomers, such as nylon 6, there is also the problem that in the original production of the polymer the molecular weight obtained sometimes is higher than was desired.